This invention relates generally to magnetic resonance imaging (MRI), and more particularly, to radio frequency (RF) coil arrays used for MRI.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person""s body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person""s body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more coils placed on or about the person.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the nucleus""location. As is well known in the art, it is possible to distinguish radiation originating from different locations within the person""s body by applying a field gradient to the magnetic field across the person""s body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Radiation of a particular frequency may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person""s body. The application of such a field gradient is also referred to as frequency encoding.
However, the application of a field gradient does not allow for two-dimensional resolution, since all nuclei at a given left-to-right position experience the same field strength, and hence emit radiation of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern radiation originating from the top versus radiation originating from the bottom of the person at a given left-to-right position. Resolution has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction to thereby perturb the nuclei in varied amounts. The application of such additional gradients is also referred to as phase encoding.
Frequency-encoded data sensed by the coils during a phase encoding step is stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
Imaging time is largely a factor of desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as xe2x80x9cparallel imaging techniquesxe2x80x9d, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. The use of multiple effective detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition.
One such multiple detector configuration is the planar strip array (PSA), in which multiple conductive microstrips are arranged in parallel within a high permittivity substrate and overlay. The planar strip array is mainly appreciated in two different areas in MRI-parallel imaging and open MRI systems. In parallel imaging, the PSA presents a way to implement a large number of element detectors. Although the conventional MRI phased array is widely used in parallel imaging, its loop structure and the decoupling methods impose some restrictions on further increasing the number of the elements in phased array. In the second area, open magnet systems, the magnetic field direction of the PSA is perpendicular to the static magnetic field unlike the circular loop coil whose magnetic field direction is parallel to the static magnetic field. Thus, conventional circular loop coils cannot excite the nuclear spins if they are laid on patient table in scanner.
The key element of PSA is transmission line resonator. Its resonant condition is that its electrical length should be either xcfx80/2 or xcfx80, which normally requires its physical length to be a quarter, or a half, wavelength of the resonant wavelength. Practically, for a 7 T whole body MRI scanner, the resonant wavelength in air is 1 m, therefore a quarter wavelength of conductor strip in air is 25 cm, which is a reasonable length for a RF detector inside MRI scanner. But for a 1.5 T MRI scanner, the resonance wavelength in air is 4.697 m, a quarter wavelength of conductor strip in air is 1.17 m, which is too long to be an effective RF detector.
Generally, the planar strip array has been used for receive mode only, due to its decoupling scheme which partially relies on low impedance (less that 2 xcexa9 preamplifiers. Since the output impedance of the transmit power amplifier is usually 50 xcexa9, the coupling among the strip array cannot be fully resolved during transmit.
What is needed is a radio frequency (RF) coil assembly, such as a planar strip array, for use in MRI systems of a given field strength. What is further needed is a transmit and receive RF coil array.
In a first aspect, a radio frequency (RF) detector array for use with a magnetic resonance imaging (MRI) system is provided. The detector array comprises a plurality of conductive array elements being substantially parallel to a conductive ground plane, a plurality of capacitors, wherein at least one capacitor is shunted from each array element to the ground plane to adjust a corresponding electrical length of each conductive strip, and, wherein a combination of each respective array element, at least one corresponding capacitor and the ground plane forms a resonator that resonates at a selected frequency.
In a second aspect, a MRI system is provided comprising a computer, a magnet assembly for generating a polarizing magnetic field, a gradient coil assembly for applying gradient waveforms to the polarizing magnetic field along selected gradient axes, and a detector array as described above for applying RF energy to excite nuclear spins of an object to be imaged, and for thereafter detecting signals generated by excited nuclei of the object to be imaged, wherein signals detected by the detector array are processed by the computer to produce MR images of the object to be imaged.